Preparation of benzopyrans and benzofurans



United States Patent BREPARATION or EBENZOPYRANS AND BENZOFURANS -0t1 i annf aqg n, many .No Drawing. Application May :2, 1956 Serial,No..582,07 3

Claims priority, application .Germany May 7, 1955 13 Claims. (Cl. 260-3461) This invention relates to a novel chemical synthesis, and to novel intermediates and processes useful ineffecting that synthesis. More particularly, the invention relates to a process of making 4,9-di(lower alkoxy)-7- methyl-furo 3.2-g] [l benzopyran-S-one, The nomenclapound khe'llin, an active principle in the seeds of A'mmi known per :se, e. g. by means of-catalytically activated hydrogem The-amino compound resulting as the product.

visnqgq.

The star-ting materials for the process of-the invention,

benzofuran, can be prepared (for example) according to the teachings inJour-nal of the American ChemicalSociety 73 1280 (1950 The nomeuclatureof these starting inater-ials is based upon'the preferred numbering indicated under System No. 841 in"The "Ring Index, supra.

'In a comprehensive embodiment, the invention provides a process which comprises reacting 3-oxo-4,7-di (lower alk-oxy)-6 hydroxy-2,B-dihydrobenzofuran with hydroxylamine thereby producing 3-oximido-4,7-di(lower alkoxy)-6-hydroxy 2,3-dihydrobenzofuran, reducing the latter thereby producing 3-amino-4,7-di(lower alkoxy)-6- hydroxy-Z,3-dihydrobenzofuran, splitting out ammonia from the latter thereby "forming 4,7-di(lower alkoxy)- 6- hydroxy-benzofuran, reacting the-latter with 'B-halo-crotonic acid lower alkyl ester thereby producing 4,7-di (lower alkoxy)-'6- l-lower' car-balkoxyl-propen-2 yloxy) benzofuran, hydrolyzing the latter thereby producing 4,7- di (lower' alkoxy) -6-( I-ca-rboxy-l-propen-2 yloxy)-benzofuranpand cyclizing the latter thereby producing 4, 9-di (lower alkoxy) 5-one. t

In a preferred, specific embodiment, the invention provides a process which comprises reacting 3-oxo-4,-7-dimethoxy-6-hydroxy-2,5-dihydrobenzofuran witlr-hydroxylamine thereby producing 3-oximido-4,7-dimethoxy-6-hydroxy-2,3-dihydrobenzofuran, reducing the latter thereby producing 3-amino-.4;7-dimethoxy 6 hydroxy-2,3-dihydrobenzofuran, splitting out ammonia from the latter thereby forining 4,7-dimethoxy 6-hydroxy benzofuran, {reacting the latter with fl-halo-crontonic acid methyl ester thereby producing 4,7-dirnethoxy- 6-(1-carbomethoxy-1-" propen-2 yloxy) -benzofuran,' hydrolyzing the latter thereb prod cing 4,7 i hqxy 6.- (i a boxy p 7 methyl furo[3.2 g] lllbenzopyranice v2 pen-2-yloxy)-benzofuran, and cycliz-ing the latter thereby producing 4,9-dimethoxy#7-methyl furo[3.2-g] [llbenzopyran-S-one.

The first stage of the process of the invention is ad-- vantageously effected by mixing the starting material, 3-oxo-4,7-di(lower alkoxy)-6 hydroxy-2,3-dihydrobenaofuran, with a hydroxylamine salt, e. g. 'hydroxylamine hydrochloride, in an inert solvent, e. g. methanol, and adding "to the reaction mixture an'acid binding-agent, e. g. sodium acetate. Preferably, moderate reaction temperatures in the range of'from about4'0" C. to about 50 C. are employed. An alternative mode of procedure comprises dissolving the start ing material in dilute aqueous sodium hydroxidetsolution, mixing the resulting solution with one molar equivalent of soda and one molar equivalent of a hydroxylamine salt, and allowing the reaction mixture (tostand at room temperature.

a The oxime of the 3-oxo starting material, produced in the first stage of the reaction, i. .e. 3-oximido-4,7-di(lower alkoxy)-6 hydroxy-2,3-dihydrobenzofuran, ,is then reduced to the corresponding amine. An appropriate mode of eifecting-t-his reduction is, for example, by reaction with sodium amalgam. Such reduction can best beetlected by suspending the oxime in an inert reaction medium, .for example, ethanol, and adding the sodium amalgam in small portions while simultaneously dropping in glacial acetic acid at such a rate that the reaction mixture is continuously maintained acidic. However, the reduction can also be effected by alternative methods of reduction of the reduction step, "i. e513 amino-4.,7-di(lower;alkoxy)- 6-hydroxy-2,3-dihydrobenzofuran, need not be isolated for further processing in the next stage.

The most advantageous method of effecting the next stage of the process of the invention, wherein ammonia is split out from the amino compound,-comprises highly concentrating the solution of salts obtained in the preceding stage, in order to drive off the organic reaction medium, and then refluxing the residue with water. This operation effects removal of ammonia,with introduction of a-new double bond between the 2 and '3-car-bon" The 4,7-'di(lower alkoxy)-6-hydroxy-benzofuran' purified by repeated distillation in vacuo.

In the next stage of the process according to the in vention, said 4,7-di-(lower alkoxy)-6-hydroxy-benzofuran; is reacted with a lower'alkyl ester of fi-halo-croton-ic acid, e.' g. a methyl or ethyl ester of 'fl-chlorojor bromocrotonic acid. The reactionis' advantageously effected" I in acetone, in the presence of potassium carbonate and at moderate reaction temperatures, e. g, in the range of fromabout 50 C. to about60 C." I

The product obtained in the preceding stage, i. e.l

4,7'-di(lower alkoxy)-6-('l-lower carbalkoxy-l-propen-z yloxy)-benz ofuran, is then saponified, according tometh ods known per se, preferably by permitting the ester to, stand at room temperature, in an aqueous or an aqueouslll alcoholic solution of an alkali'metal hydroxide, for several hours, or for a longer period ranging up to several days. The free acid, 4,7-di(lower alkoxy)-6-(l-carbox yl-propen-Z yloxy)-benzoturan, is then obtained from the saponifieation' mixture by acid.

acidification with a mineral In the final stage of the process according to the invention, the above identified free acid is cyclized. A suitable mode of procedure, for example, comprises pouring acetyl chloride over the free acid and then adding a trace of concentrated sulfuric acid or perchloric acid, thereby effecting solution of the reactants. The cyclic reaction product separates from the reaction mixture after the latter stands for some time. A preferred method of effecting the cyclization reaction comprises permitting the reaction mixture to stand at room temperature fora period of from about two to about ten days. The product is formed slowly, and as the amount thereof gradually increases the material eventually crystallizes from the reaction mixture. Presumably the precipitated material is an oxoniurn salt. The precipitated material can be converted to the free 4,9-di(lower alkoxy) 7- methyl-furo[3.2-g][llbenzopyran-S-one by reacting the precipitate with water or with sodium bicarbonate solution.

The invention is further disclosed in the following examples, which are illustrative but not limitative thereof.

Example I To 750 ml. of pure methanol 42 g. of 3-oxo-4,7-dirnethoxy-6-hydroxy-2,3-dihydrobenzofuran was added and dissolved to the maximum extent possible; and tothemixture was added, while stirring, a solution of 36 g. of hydroxylamine hydrochloride in a little water and then 48g. of finelypowdered anhydrous sodium acetate was added. The reaction mixture was stirred for 10 hours' at 40 to 50 C., and then for a further period of 10 hours at room temperature. The insoluble oxime was'filtered on with suction and the filter cake was triturated with water in order to dissolve the inorganic materialicontained therein. The methanolic filtrate was concentrated in vacuo, and the concentrated methanolic solutionwas mixed with water, thereby precipitating a further quantity of the oxime. In this manner there was obtained ;37 g. of 3-oxirnido-4,7-dimethoxy-6-hydroxy-2,3-dihydrobenzofuran, which was recrystallized from ethanol (alternatively, from aqueous methanol). Decomposition point, 198 C.'(with previous carbonization); Z ill l Example '2 .to maintain the reaction mixture constantly acidic.

The reaction mixture was then stirred l2 hourslonger at room temperature. At the, end of this time, the mercury was separated and the precipitated salts were dissolved by addition of water. The mercury was purified, first by shaking with water and then with acetone, The combined solutions, which contained 3-amino-4,7- dimethoxy-6-hydroxy-2,3-dihydrobenzofuran in salt form, were concentrated in vacuo almost to dryness, then 500 ml. of water was added and .the mixture was refluxed for two hours. The solution was cooled, and extracted. with diethyl ether. The ethereal layer was separatedand dried over sodium sulfate. The solvent was driven off and the residual oil was distilled twice in vacuo, thereby yielding 14 g. of 4,7:dimethoxy-6-hydroxy-benzofuran in the form of an orange colored oil having B. P. 145 C./l.2 mrn.; n =1.5721.

Example 3 A solution was prepared 'by dissolving 1.94 g. of 4,7-dimethoxy-6 hydroxy-benzofuranin 50 ml. of anhydrous acetone and the solution was mixed, while stirring, with 10 g. of anhydrous, finely powdered potassium carbonate. 1.34 g. of B-chloro-crotonic acid methyl ester was added dropwise and the reaction mixture was heated at 50 to 60 C. for 12 hours, while stirring. The solution was filtered from the inorganic salts while still hot. The filter cake was washed with warm acetone, the washings being added to the filtrate, and the combined liquors were concentrated. The residual crude 4,7-dimethoxy- 6-( l-carbomethoxy-l-propen 2 yloxy)-benzofuran was dissolved in 13 ml. of ethanol. In order to saponify the ester, 6.5 ml. of aqueous sodium hydroxide solution containing 50% by weight NaOH was added, and the mixture was allowed to stand for five days at room temperature, with occasional shaking.- The clear, orange colored solution so obtained was acidified with concentrated hydrochloric acid while cooling with ice. The precipitate was recrystallized from methanol with addition of a little activated carbon. There. was thus obtained 4,7-dimethoxy-6-( l-carboxy-l-propen 2 yloxy) -benzofuran in the form of silver-white needles having a decomposition point of 189 C. (with decarboxylation).

First, 25 ml. of acetyl chloride was poured over 2.78 g.

.. of 4,7-dimethoxy-6-(l-carboxy-1-propen-2-yloxy)-benzofuran and then 5 drops of concentrated sulfuric acid was added, whereupon all of the starting material went into solution, resulting. in a red-brown coloration. Upon standing for 24 hours, the first crystalline precipitate was observed. The mixture was permitted to, stand for 10 days at room temperature, then the precipitated product was filtered off with suction and was suspended in aqueous sodium bicarbonate solution. The filtrate was concentrated in vacuo and was slowly mixed, while cooling with ice, with a saturated aqueous sodium bicarbonate solution. From the thus neutralized solution, fine needles of M. P. l50 -151 crystallizedin about 12 hours; The combined crystalline precipitates were recrystallizedfrom aqueous ethanol and decolorized with activated carbon. The yield of khellin (4,9-dimethoxy-7-methyl-furo[3.2-g] [1 benzopyran-S-one), having a melting point of 154 C., amounted to at least 800 mg. I The mixedmelting point with natural khellin showed no depression.

I claim:

1. A process which comprises reacting 3-oxo-4,7-di-R- 6-hydroxy-2,Ii-dihydrobenzofuran with hydroxylamine thereby producing 3-oximido-4-7-di-R-6-hydroxy-2,3-dihydrobenzofuran, reducing the latter thereby producing 3-amino-4,7-di-R-6-hydroxy-2,3 dihydrobenzofuran, splitting out ammonia from the latter thereby producing 4,7-di-R-6-hydroxy-benzofuran, reacting the latter with fl-halo-crotonic acid lower alkyl ester thereby producing 4,7-di-R-6-(l-lower carbalkoxy-l-propen-Z-yloxy)benzofuran, hydrolyzing the latter thereby producing 4,7-di- R-6-(1-carboxy-l-propen-Z-yloxy) -benzofuran, and cyclizing the latter thereby producing 4,9-di-R-7-methylfuro[3.2-g]lllbenzopyran-S-one; R representing a lower alkoxy radical.

2. A process which comprises cyclizing 4,7-di-R-6(1- carboxy-l-propen-2-yloxy)-benzofuran by treatment with acetyl chloride thereby producing 4,9-di-R-7-methyl-furo [3.2-g][llbenzopyran-S-one; R representing a lower.

hydroxy-benzofuran with a lower alkyl ester-of B-halo-' crotonic acid and hydrolyzing; R representing a lower alkoxy radical. Y

6. 4,7-di-R-6-hydroxy-benzofuran; R lower alkoxy radical. V 1

7.A' process of making 4,7-di-R-6-hydroxy-benzofuran which comprises deaminating 3-amino-4,7-di-R-6-hydroxyrepresenting a 5 2,3-dihydrobenzofuran; R representing a lower alkoxy radical. Y

8. 3-amino-4,7-di-R-6-hydroxy-2,S-dihydrobenzofuran; R representing a lower alkoxy radical.

9. 3-amino-4,7-dimethoxy-6-hydroxy-2,3-dihydrobenzofuran.

10. A process of making 3-amino-4,7-di-R-6-hydroxy-2, 3-dihydrobenzofuran which comprises reducing 3-oximido-4,7-di-R-6-hydroxy-2,3-dihydrobenzofuran; R representing a lower alkoxy radical.

11. 3-oximido-4,7-di-R-6-hydroxy-2,3-dihydrobenzofufan; R representing a lower alkoxy radical.

12. 3-oximido-4,7-dimethoxy-6-hydroxy-2,3-dihydrobenzofuran.

13. A process of making 3-oximido-4,7-di-R-6-hydroxy- 2,3-dihydrobenzofuran which comprises reacting 3-oxo- 4,7-di-R-6-hydroxy-2,3-dihydrobenzofuran with a hydroxylamine-yielding compound; R representing a lower alkoxy radical.

Chemical Reviews, vol. 48, page 558 (1951); Elderfield, R. C., Heterocyclic Cmpds., vol. 2 (1951), pages 42, 67, 248.

UNITED STATES PATENT OFFICE Certificate of Correction Patent No. 2,855,406 October 7, 1958 Otto Damn It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, line 7, for Claims priority, application Germany May 7, 1955 read Claims priority, application Germany May 17, 1955.

Signed and sealed this 28th day of April 1959.

Attest: T. B. MORROW, ROBERT C. WATSON,

Attestz'ng Ofiicer. Commz'ssz'oner of Patents. 

1. A PROCESS WHICH COMPRISES REACTING 3-OXO-4,7-DI-R6-HYDROXY-2,3-DIHYDROBENZOFURAN WITH HYDROXYLAMINE THEREBY PRODUCING 3-OXIMIDO-4-7-DI-R-6-HYDROXY-2,3-DIHYDROBENZOFURAN, REDUCING THE LATTER THEREBY PRODUCING 3-AMINO-4,7-DI-R-6-HYDROXY-2,3-DIHYDROBENZOFURAN, SPLITTING OUT AMMONIA FROM THE LATTER THEREBY PRODUCING 4,7-DI-R-6-HYDROXY-BENZOFURAN, REACTING THE LATTER WITH B-HALO-CROTONIC ACID LOWER ALKYL ESTER THEREBY PRODUCING 4,7-DI-R-6-(1-LOWER CARBALKOXY-1-PROPEN-2-YLOXY) BENZOFURAN, HYDROLYZING THE LATTER THEREBY PRODUCING 4,7-DIR-6-(1-CARBOXY-1-PROPEN-2-YLOXY)-BENZOFURAN, AND CYCLIZING THE LATTER THEREBY PRODUCING 4,9-DI-R-7-METHYLFURO(3.2-G)(1)BENZOPYRAN-5-ONE; R REPRESENTING A LOWER ALKOXY RADICAL. 